Alarm

ABSTRACT

An alarm for detecting radiation and/or pollutants such as smoke, carbon monoxide or the like has a housing ( 500 ), an alarm circuit ( 400 ) including a detector (DET 1 ) for detecting said radiation and/or pollutants, first electrical connections (PL 1,  PL 2 ) connectable to an external power supply for supplying power to the alarm circuit, and a control circuit ( 300 ) responsive to receipt of a preselected number of pulses over a preselected time period to apply a preset control signal to the alarm circuit ( 400 ). The alarm circuit ( 400 ) is responsive to the preset control signal to reset or test the alarm in dependence on the preset control signal.

The present invention relates to an alarm and particularly, but notexclusively, to an improved form of mains-powered smoke alarm.

Until recently, smoke alarms and other types of alarms for detectingradiation, heat and air pollutants or the like, were relatively bulkydevices powered only by means of a battery. No provision for rechargingthe battery was included and thus correct operation of the alarmrequired the regular changing of the battery to ensure the alarmremained powered. Owing to an increasing awareness of the need for suchalarms in domestic buildings and offices, it has become common toprovide alarms which are mains-powered and which include a rechargeablebattery for powering the alarm in the event that mains power isdisrupted.

An improvement to general mains powered smoke alarms are alarms whichcan be connected to a lighting circuit. International Patent Applicationpublication No. WO0/21407 discloses an alarm for detecting radiationand/or pollutants such as smoke, carbon monoxide or the like which isarranged to interconnect between a light fitting and a light source suchas a bulb. The alarm is powered by the light fitting when the lightfitting is energised and is powered by a battery when the light fittingis de-energised.

International Patent Application publication No. WO 00/58924 disclosesan alarm for detecting radiation and/or pollutants such as smoke, carbonmonoxide, methane, radon or the like comprising a housing which isintended to replace a ceiling rose for a light fitting.

The above described devices permit relatively easy installation toexisting lighting circuits but suffer the disadvantage that a lightfitting, such as a batten or pendant ceiling fitting, is required forsuch installation. It is difficult or impossible to install such devicesat locations in a building where there are no light fittings. Buildingregulations currently often require that mains-powered alarms be fittedat specific areas within buildings which may not coincide with theposition of light fittings.

It is an aim of the present invention to provide an improvedmains-powered alarm connectable in a lighting circuit or other mainscircuit. It is a further aim of the invention to provide an alarm whichis more easily installed and which does not require to be fitted inconjunction with a light fitting.

The present invention provides an alarm for detecting radiation and/orpollutants such as smoke, carbon monoxide or the like having: a housingmeans; an alarm circuit including detection means for detecting saidradiation and/or pollutants; first electrical connection meansconnectable to an external power supply for supplying power to saidalarm circuit; and control means responsive to receipt of a preselectednumber of pulses over a preselected time period to apply a presetcontrol signal to said alarm circuit; wherein said alarm circuit isresponsive to said preset control signal to reset or test said alarm independence on said preset control signal.

In a preferred form of the invention said control means is responsive tothe energising and deenergising of the external power supply saidpreselected number of times over said preselected time period to applysaid preset control signal to said alarm circuit. Said alarm has firstswitch means actuable by a user to generate a respective pulse for eachactuation thereby to apply a user selected number of pulses to saidcontrol means; and said control means is responsive to receipt of saidpreselected number of said pulses over said preselected time period toapply a preset control signal to said alarm circuit.

Preferably, said first switch means is mounted on said alarm housing.

Preferably, said first switch means is mounted remote from said alarmhousing.

Preferably, said first switch means is adapted for connection to aswitch live side of a switch for a lighting circuit.

Preferably, said alarm has second electrical connection means forconnection to a switch live side of a switch for a lighting circuit; andwherein said second electrical connection means is operable to receivepulses caused by user actuation of said switch between its on and offstates and apply said pulses to said control means thereby to cause apreset control signal to be applied to said alarm circuit in response togeneration of said preselected number of pulses over said preselectedtime period.

Preferably, switch means for an external light source are provided andare actuable in response to generation of a preselected control signalto energise said light source.

Preferably, the alarm comprises a relay and a light source wherein saidrelay is actuable in response to generation of a preselected controlsignal to energise said light source.

Preferably, when said preselected number of pulses over said preselectedtime period is one, said control means is operable to apply a presetcontrol signal to said alarm circuit thereby to reset said alarm.

Preferably, when said preselected number of pulses over said preselectedtime period is one, said control means is operable to apply a presetcontrol signal to said alarm circuit thereby to test said alarm.

Preferably, when said preselected number of pulses over said preselectedtime period is two, said control means is operable to apply a presetcontrol signal to said alarm circuit thereby to test said alarm.

Preferably, when said preselected number of pulses over said preselectedtime period is two, said control means is operable to apply a presetcontrol signal to said alarm circuit thereby to reset said alarm.

Preferably, said alarm circuit comprises means for reducing thesensitivity of said detection means.

Preferably, said means for reducing the sensitivity of said detectionmeans is operable in response to generation of a reset control signal bysaid control means to reduce the sensitivity of said detection means fora preselected time period thereby to reset said alarm.

Preferably, said alarm circuit comprises means for increasing thesensitivity of said detection means.

Preferably, said means for increasing the sensitivity of said detectionmeans is operable in response to generation of a test control signal bysaid control means to increase the sensitivity of said detection meansfor a preselected time period thereby to test said alarm.

Preferably, the alarm comprises a battery for supplying power to saidalarm in the absence of mains power, and a charging circuit includingsaid first electrical connection means for supplying power to a powerrail for said alarm and for charging said battery.

Preferably, the alarm comprises isolating means for selectablyelectrically disconnecting said battery from said alarm thereby tominimise leakage from said battery when said alarm is inactive.

Preferably, said isolating means comprises a second switch means in saidpower rail switchable between a first, conducting state connecting saidbattery to said alarm and a second, non-conduction state disconnectingsaid battery from said alarm.

Preferably, said charging circuit comprises a third switch meansswitchable between a first, conducting state and a second,non-conducting state in dependence on the voltage on said power rail;and wherein: when said third switch means is in said first, conductingstate said third switch means is operable to retain said isolatingsecond switch means in its conducting state; and when said third switchmeans is in said second, non-conducting state the state of said thirdswitch means is dependent on the voltage on said power rail such thatsaid second switch means is non-conducting in response to said voltageon said power rail being below a preselected value indicating a lowbattery charge, thereby to disarm said alarm during charging of saidbattery.

Preferably, the alarm comprises a disconnect means actuable to switchsaid switch means into its non-conducting state thereby disabling saidswitch means and preventing actuation of said alarm.

Preferably, said disconnect means comprises button means movable betweena first, OFF position wherein said switch means is renderednon-conducting and a second, ON position wherein said switch means isenabled.

Preferably, said switch means is a multi electrode semiconductor devicehaving a control electrode for controlling conduction between furtherelectrodes thereof; and said button means is movable into its first, OFFposition to vary the potential on said control gate means thereby torender said switch means non-conducting.

Preferably, said housing comprises: a first backing plate for mountingon a surface; a second backing plate detachably mountable on said firstbacking plate; and a cover means for covering said backing plates; andwherein the arrangement of said disconnect means is such that engagementof said second backing plate on said first backing plate moves saiddisconnect means into its second, ON position thereby to enable saidswitch means and disengagement of said second backing plate from saidfirst backing plate moves said disconnect means into its first, OFFposition thereby to disable said switch means.

Preferably, the alarm comprises indicator means operable in response topower on said voltage rail downstream of said isolating means toindicate that said alarm is enabled.

The present invention also provides an alarm for detecting radiationand/or pollutants such as smoke, carbon monoxide or the like having: ahousing means; an alarm circuit including detection means for detectingsaid radiation and/or pollutants; first electrical connection meansconnectable to an external power supply for supplying power to saidalarm circuit; and switch means for a light source, said switch meansbeing actuable in response to triggering of said alarm to energise saidlight source.

Preferably, said switch means comprises a relay and said light source isexternal to said alarm.

Preferably, said light source is mounted in said alarm.

The present invention further provides an alarm for detecting radiationand/or pollutants such as smoke, carbon monoxide or the like having: ahousing means; an alarm circuit including detection means for detectingsaid radiation and/or pollutants; first electrical connection meansconnectable to an external power supply for supplying power to saidalarm circuit; a battery for supplying power to said alarm in theabsence of mains power; a charging circuit including said firstelectrical connection means for supplying power to a power rail for saidalarm and for charging said battery; and an isolating means forselectably electrically disconnecting said battery from said alarmthereby to minimise leakage from said battery when said alarm isinactive.

Preferably, said isolating means comprises a second switch means in saidpower rail switchable between a first, conducting state connecting saidbattery to said alarm and a second, non-conduction state disconnectingsaid battery from said alarm.

Preferably, said charging circuit comprises a third switch meansswitchable between a first, conducting state and a second,non-conducting state in dependence on the voltage on said power rail;and wherein: when said third switch means is in said first, conductingstate said third switch means is operable to retain said isolatingsecond switch means in its conducting state; and when said third switchmeans is in said second, non-conducting state the state of said thirdswitch means is dependent on the voltage on said power rail such thatsaid second switch means is non-conducting in response to said voltageon said power rail being below a preselected value indicating a lowbattery charge, thereby to disarm said alarm during charging of saidbattery.

Preferably, the alarm comprises a disconnect means actuable to switchsaid switch means into its non-conducting state thereby disabling saidswitch means and preventing actuation of said alarm.

Preferably, said disconnect means comprises button means movable betweena first, OFF position wherein said switch means is renderednon-conducting and a second, ON position wherein said switch means isenabled.

Preferably, said switch means is a multi electrode semiconductor devicehaving a control electrode for controlling conduction between furtherelectrodes thereof; and said button means is movable into its first, OFFposition to vary the potential on said control gate means thereby torender said switch means non-conducting.

Preferably, said housing comprises: a first backing plate for mountingon a surface; a second backing plate detachably mountable on said firstbacking plate; and a cover means for covering said backing plates; andwherein the arrangement of said disconnect means is such that engagementof said second backing plate on said first backing plate moves saiddisconnect means into its second, ON position thereby to enable saidswitch means and disengagement of said second backing plate from saidfirst backing plate moves said disconnect means into its first, OFFposition thereby to disable said switch means.

Preferably, the alarm further comprises indicator means operable inresponse to power on said voltage rail downstream of said isolatingmeans to indicate that said alarm is enabled.

The present invention will now be described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a block circuit diagram of a preferred form of alarm accordingto the invention;

FIG. 2 is a schematic circuit diagram of a charging circuit of the alarmof FIG. 1;

FIG. 3 is a schematic circuit diagram of a disconnect circuit of thealarm of FIG. 1;

FIG. 4 is a schematic circuit diagram of a control circuit of the alarmof FIG. 1;

FIG. 5 is a schematic circuit diagram of a detection circuit of thealarm of FIG. 1;

FIG. 6 is a circuit diagram of an alternative form of charging circuitfor the alarm of FIG. 1;

FIG. 7 is a circuit diagram of an alternative form of control circuitfor the alarm of FIG. 1;

FIG. 8 is a first perspective view of a housing for the alarm of FIG. 1;

FIG. 9 is a second perspective view of the housing of FIG. 8;

FIG. 10 is a partial section through the alarm of FIG. 8;

FIG. 11 is a perspective view from above of a mechanical disconnectmechanism for the disconnect circuit of FIG. 3;

FIG. 12 is a further perspective view from above of the mechanicaldisconnect mechanism of FIG. 11;

FIG. 13 is a perspective view from below of part of the mechanicaldisconnect mechanism of FIG. 11;

FIG. 14 is a perspective view from below of part of the alarm housingshowing a power socket of the alarm and a socket holder in spacedrelationship;

FIG. 15 is a perspective view similar to that of FIG. 14 showing thepower socket engaged in the socket holder;

FIG. 16 is a schematic diagram showing a first method of connection ofthe alarm to the consumer wiring system;

FIG. 17 is a schematic diagram showing a second method of connection ofthe alarm to the consumer wiring system; and

FIG. 18 is a schematic diagram showing a third method of connection ofthe alarm to the consumer wiring system.

FIG. 19 is a block diagram of the circuit of a further form of alarm;and

FIG. 20 is a schematic diagram of a power on circuit for the disconnectcircuit of FIG. 3.

While the following description is made with reference to a smoke alarm,it will be understood that the invention is applicable to other types ofalarms, such as those for detecting radiation, air pollutants such asmethane, radon or carbon monoxide, and/or heat or the like. In addition,the term “earth” in the context of a voltage or potential is used in thefollowing description conveniently to refer to a reference or signalearth potential, which may or may not be equal to true earth potential,and no limitation to zero volts or true earth potential is intended.

It should also be noted that the symbol Vcc is used to indicate aconnection to a supply rail of the alarm circuit whilst the symbol of aninverted triangle is used to represent a connection to an earth rail ofthe circuit.

Referring firstly to FIG. 1, this shows a block circuit diagram for apreferred form of alarm according to the invention. The alarm circuithas a charging circuit 100, an isolating or disconnect circuit 200, acontrol circuit 300, an alarm detection circuit 400 and a power onwarning circuit 800.

The charging circuit provides a rectified and smoothed voltage for thecontrol and detection circuits 300, 400 whilst the disconnect circuit200 controls the application of the supplied voltage to the control anddetection circuits 300, 400.

The circuits 100 to 800 will be well understood by those skilled in theart and for convenience, therefore, only those features of the circuitswhich are important for the understanding (and not necessarily theoperation) of the invention are described in detail.

The charging circuit 100 is shown in detail in FIG. 2 and includes firstand second inputs PL1, PL2 for connection to the live and neutral cablesof an AC power supply. In the illustrated embodiment, the AC powersupply is formed by the live and neutral cables of an existing mainslighting or ring circuit such as may be found in domestic or officebuildings. The first input PL1 is connected to the switched live cablefor the lighting circuit so that power is only supplied to the chargingcircuit 100 when the light is switched on. The phrase “switched live” asused herein refers to the cable which connects the light switch of thelighting circuit to a lamp of the circuit such that when the switch isclosed, power is applied through the cable to the lamp.

The first and second inputs PL1, PL2 of the charging circuit areconnected to respective inputs of a diode rectifier or rectifier bridgeBR1 which serves to apply full-wave rectification to the AC voltage,thereby generating a DC voltage.

The outputs of the rectifier bridge BR1 form positive and earth rails110, 112 for the charging circuit 100. The rectified DC voltage isapplied to the positive rail 110 and a smoothing capacitor C2 isconnected between the positive and earth rails 110, 112 for smoothingthe DC current from the rectifier bridge BR1. A Zener diode 108 isreverse biased across the positive and earth rails 110, 112 for clippingthe voltage output of the rectifier bridge BR1 and hence isolating thefurther circuitry in the charging circuit from voltage spikes on thepower supply.

The DC voltage from the bridge rectifier BR1 is applied to the input ofa voltage regulator IC1 which serves to regulate the voltage. The outputof the voltage regulator forms a charging rail 111 and the referenceinput of the voltage regulator is connected to the junction between tworeference resistors R7, R8, connected in series between the chargingrail 111 and the earth rail 112.

The charging circuit further includes a switch in the form of atransistor TR5 whose collector is connected to the charging rail 111 viaa resistor R31. The emitter of the transistor TR5 is connected to theearth rail 112 and the base is connected to a potential divider formedby two resistors R38, R39 connected in series between the charging andearth rails 111, 112. The purpose of the transistor TR5 is describedbelow.

FIG. 3 illustrates the disconnect circuit 200. The disconnect circuit200 is connected to the charging circuit 100 via the charging rail 111at point C and to the collector of the transistor TR5 at point B. Thedisconnect circuit 200 includes a rechargeable cell or battery B1, thepositive terminal of which is connected to the charging rail 111 via aparallel combination of a resistor R30 and a Schottky diode D9. Thenegative terminal of the cell B1 is connected to the earth rail 112. Thecharging rail 111 is connected to the source of a P-type Field EffectTransistor (FET) TR3, the drain of which is connected to and forms asupply rail 210 for the remaining circuitry of the alarm as describedbelow.

The gate of the FET TR3 is connected, via a limiting resistor R40 to thecollector of the transistor TR5 at point B. In addition, the source andgate of the FET TR3 are arranged to be connected together ordisconnected by means of a connection arrangement 550. The connectionarrangement 550 may be of any suitable type which permits the easy andselective connection and disconnection of the source and gates of theFET TR3. For example, it may be achieved by a fuse-type connector, ajumper or even a manual switch. An important element of this feature isthat the source and gate of the FET TR3 are quickly and easily connectedor disconnected by a user of the smoke alarm. A preferred form ofconnection arrangement is described in detail below with reference toFIGS. 11 to 13.

Operation of the recharging and disconnect circuits 100, 200 will now bedescribed. The AC voltage from the mains supply is applied to the inputsPL1, PL2 and the alternating current is full-wave rectified to a DCsignal by means of the diode bridge BR1. The DC voltage across thepositive and earth rails 110, 112 is smoothed by means of the smoothingcapacitor C2 and is regulated by the voltage regulator IC1. Duringperiods when the charging circuit is operable (i.e. while the AC voltageis applied to the inputs PL1, PL2) a DC voltage is applied to the baseof the transistor TR5 which is thus switched on.

With the transistor TR5 switched on, the potential at the collector ofthe transistor TR5 is “pulled down” to approximately the potential onthe earth rail 112 thus pulling down the gate of the FET TR3 which isconnected to the collector of TR5. Since the FET TR3 is a P-type device,a relatively low potential applied to the gate thereof causes the FETTR3 to switch on. Power from the charging circuit 100 is thus suppliedvia the voltage regulator IC1 and the FET TR3 to the supply rail 210 fordistribution to the further circuitry of the alarm. In addition, currenton the charging rail 111 flows through the resistor R30 for charging therechargeable battery B1.

It is envisaged that the input PL1 may be connected to the switched livecable of, for example, a lighting circuit for a light bulb (not shown)so that power will be applied to the charging circuit 100 from thelighting circuit when the lighting circuit is switched on. Duringperiods when the lighting circuit is de-energised (i.e. the light isswitched off), power to the supply rail 210 is provided by means of therechargeable battery B1. Since, during such periods, no power issupplied to the inputs PL1, PL2, the potential on the charging rail 111is substantially the same as that on the earth rail 112.

Since the potential applied to the collector of the transistor TR5, andhence to the gate of the FET TR3, is low, the latter remains switched oneven through the transistor TR5 is switched off. Current is thussupplied to the further circuitry of the alarm from the battery B1 viathe FET TR3.

It will be understood that there may be some circumstances in which thecharging circuit is not be used for some considerable time. One suchcircumstance is when the alarm is in transit (i.e. before installation)or during shipping from manufacturer to retailer. In thesecircumstances, clearly, no charging current is available and the batterycontinues to provide power to the alarm even though the alarm is notrequired to be operative. As a result, over a period of time the batteryB1 will lose its charge. While this is acceptable in some circumstances,it would be advantageous to reduce the current drain from the battery toa minimal level.

A solution to this problem as provided by the present invention is toenable the battery B1 to be selectively disconnected from the remainingcircuitry of the alarm in order to eliminate or minimise the currentdrain from the battery. This is achieved through the connection means550. The connection means 550 is connected across the charging rail 111and the gate of the FET TR3 and is arranged selectively to connect thesource of the FET TR3 to the gate thereof. In this state, the FET TR3 iseffectively shorted out and the voltage applied to the gate rises fromearth potential to a level close to that provided on the charging rail111 by the battery B1.

This raised voltage on the gate causes the FET TR3 to switch off therebypreventing current flow from the battery B1 to the remainder of thecircuitry. It will be noticed from FIGS. 2 and 3 that current paths fromthe battery B1 still exist through the resistors R40, R31 and then viaR7, R8 and R38, R39. However, the limiting resistor R40 preferably has aresistance in the order of MegaOhms, which is sufficiently high toreduce significantly the current flow from the battery B1.

Advantageously, the connection arrangement 550 may be arranged so thatthe source and gates of the FET TR3 are shorted by default until suchtime as the alarm is installed, as described later.

It will be understood that the above described mechanism enables thebattery B1 to retain a usable charge for a considerable length of timebefore it is required to be recharged. Thus, alarms which are shippedwith their batteries installed will still retain sufficient charge to beoperable after sale by the retailer. This avoids the common problemwhereby mains powered alarms which employ rechargeable batteries as backup power supplies are unable to charge the battery if the charge on thebattery drops below a certain level as described below.

Another circumstance in which the charging circuit may not be used forsome considerable time is when, after installation of the alarm, thelighting circuit is not energised for a long period of time. In thesecircumstances no recharging power is available and the smoke alarmcircuitry is powered solely by the battery B1.

A problem with conventional smoke alarms is that, when the charge on thebattery drops below a predetermined level, the operation of the alarmcan become unstable and unpredictable and the alarm will often revert toa constant alarm condition. In this case, switching on the lightingcircuit in order to charge the battery may be unsuccessful since theextra current required to power the triggering alarm may exceed thatavailable or required to charge the battery. There will thus be littleor no current available to charge the battery and the circuit willcontinue to alarm or “bog down”, thus preventing the battery from beingcharged.

Conventional smoke alarms do not possess any means to prevent this andoften require the removal of the rechargeable battery and theindependent recharging thereof. However, for devices havingnon-removable rechargeable batteries, it is not possible to recharge thebattery and the alarm as a whole may have to be discarded.

The alarm of the present invention addresses this problem by means ofthe connection arrangement 550. By connecting the source and gate of theFET TR3 together, the FET TR3 is switched off and the battery B1 iseffectively disconnected from the remaining circuitry of the smokealarm, as described above. As such, there is no drain from the batteryto the alarm circuitry and substantially all of the current availablefrom the charging circuitry can be used to recharge the battery.

It will be appreciated that this solution requires a positive action onthe part of the user, i.e. the manual operation of the connectionarrangement 550, to enable the battery to recharge. In addition, itrequires that the connection arrangement 550 be capable of beingswitched between closed and open positions selectively and repetitively.

A second solution to this problem is provided for by means of thetransistor TR5 which effectively permits automatic disconnection of thebattery from the remaining circuitry of the alarm when the charge on thebattery B1 falls below a predetermined level.

During periods when the lighting circuit is not energised, and hence thecharging circuit is inoperable, the charge on the battery will graduallyreduce. The transistor TR5 remains switched off since the potentialapplied to the base thereof is low (a blocking diode D3 prevents currentfrom the battery B1 raising the potential to a level sufficient toswitch the transistor TR5 on). In addition, the FET TR3 remains on,irrespective of the charge on the battery, since the potential appliedto the gate of the FET TR3 (determined by the potential on the chargingrail 111) is low.

When the recharging circuit is switched on (i.e. the lighting circuit isenergised) the voltage on the charging rail 111 increases. However,owing to the large current required to charge the battery and thus drawnby the battery the voltage on the charging rail 111 does not reach alevel sufficient to switch the charging transistor TR5 on. Nevertheless,the voltage on the charging rail 111 does rise sufficiently to raise thepotential applied to the gate of the FET TR3 to a level sufficient toswitch the FET TR3 off, thereby disconnecting the battery B1 from thefurther circuitry of the alarm. This permits almost all of the currentfrom the charging circuit to be used to charge the battery.

As the charge on the battery rises, the current drawn by the batterydecreases and the voltage on the charging rail 111 increases. When thevoltage on the charging rail 111 exceeds a predetermined level, thetransistor TR5 is switched on and the potential applied to the gate ofthe FET TR3 is pulled down to the potential on the earth rail 112,thereby switching on the FET TR3 and reconnecting the battery B1 to thefurther circuitry of the alarm.

Whilst the provision of the transistor TR5 enables the alarm to berecharged even when the battery is fully drained, i.e. has substantiallyzero charge, without user intervention, it is envisaged that there maybe occasions when the user may wish to disconnect the power supply fromthe detection circuitry of the alarm in order to disable the alarm, forexample to permit the alarm to be moved to a new location.

To address this problem, the connection arrangement 550 is preferablyarranged to be easily accessible by the user and to be repeatedlyconnected and disconnected thereby to short out the FET TR3 and hencedisconnect the detection circuitry from the power supply as describedabove.

In order to avoid the situation where the disconnect circuit isoperational and the alarm isolated without a user realising, a power oncircuit 800 is provided for the disconnect circuit 200 as shown in FIG.20. The gate of a FET TR10 is connected to the power rail 210 on thealarm side of the transistor TR3 (FIG. 3) by a resistor R92 with thesource connected to earth via a light emitting diode or other lightsource LED1. The drain is connected to the power rail 111 on thecharging circuit side of the disconnect circuit by way of resistor R91.Power must be present both on the supply rail 111 and the alarm rail 210before the LED1 will light and indicate that the alarm is operational.

A further problem with existing alarms is the frequent occurrence offalse alarms caused by, for example, cooking fumes, controlled firessuch as coal or gas fires or cigarettes or the like. Alarms whichfrequently trigger falsely are often removed or disabled by the user insome way. Clearly, where it is possible to deactivate a smoke alarm, forexample by removing the battery or operating a switch, this can bepotentially highly dangerous should a real fire occur during the periodthe alarm is switched off, regardless of whether the alarm is switchedoff indefinitely or for a predetermined period of time.

To address this problem, the present invention provides a unique resetfunction which enables the alarm to be reset following a false alarmwithout causing the alarm to be switched off. Moreover, this resetfunction is effected simply and easily merely by flicking on and off aswitch on the alarm itself or the light switch of the lighting circuitto which the alarm is connected a preset number of times over a presettime period.

FIG. 4 illustrates a control circuit 300 which responds to pulses on aninput rail 301 which is connected at A to the positive rail 110. Thepulses may thus be provided by the energising and de-energising of thelighting circuit to which the alarm is connected, i.e. by flicking thelight switch a preset number of times over a preset time period.

The control circuit 300 includes a first integrated circuit IC2 (shownfor convenience as two separate blocks IC2-A, IC2-B in FIG. 4) which isa dual precision monostable integrated circuit. IC2 provides arespective output pulse for each on/off flick of the light switch. Theoutput of IC2 is connected to a second integrated circuit IC3 which is acounter integrated circuit. IC3 has a first output connected to a firstoutput rail 306 and is arranged to apply a voltage to the first outputrail 306 in response to a single output pulse from IC2 representing asingle energising and de-energising of the lighting circuit (i.e. asingle on/off flick of the light switch). IC3 also has a second outputconnected to a second output rail 308 and is arranged to apply a voltageto the second output rail 308 in response to two successive outputpulses of IC2 representing a double energising and de-energising of thelighting circuit (i.e. two on/off flicks of the light switch). The firstand second output rails 306, 308 are connected to the detection circuit400 shown in FIG. 5 at points E and F, respectively.

Referring to FIG. 5, the detection circuit 400 of the alarm includes adetector integrated circuit IC4 such as a Motorola MC145018 low-powercomplementary MOS ionisation smoke detector integrated circuit. Thedetector integrated circuit IC4 includes an ionisation chamber DET1which is connected between the supply rail (shown as Vcc) and the earthrail via a limiting resistor R20 and which generates a normal operatingvoltage Vno which is applied to a detector input 402 of the detectorintegrated circuit IC4.

The ionisation chamber DET1 is arranged such that when smoke isdetected, the voltage Vno, generated by the ionisation chamber andapplied to the detector input of the detector integrated circuit IC4,drops. The detector integrated circuit IC4 has a predetermined butadjustable sensitivity level which is set by means of a referencevoltage Vref applied to a sensitivity input 404 of the detectorintegrated circuit IC4. When the voltage Vno applied to the detectorinput of the detector integrated circuit 104 by the ionisation chamberDET1 drops below Vref, the alarm triggers.

One electrode of a capacitor C13 is connected to a point between thelimiting resistor R20 and the ionisation chamber DET1 and also to thecollector of a first detector transistor TR2. The other electrode of thecapacitor C13 is connected to the earth rail 112. The emitter of thefirst detector transistor TR2 is connected to the earth rail 112 whilstthe base thereof is connected to the first output rail 306 at point E.

If the lighting circuit is energised and de-energised once within apredetermined time period determined by the time constant of an R-Ctimer circuit associated with IC2, the pulses on the input line 301 aredetected by IC2 which sends a control signal to the counter integratedcircuit IC3. On receiving the control signal, the counter integratedcircuit IC3 applies a voltage to the first output rail 306 which is thenapplied to the base of the first detector transistor TR2. The firstdetector transistor TR2 is thus switched on.

Current then flows from the supply rail 210, through the first detectortransistor TR2 and the voltage applied to the ionisation chamber ispulled down to a relatively low potential. In addition, the capacitorC13 discharges through the first detector transistor TR2. As a result,the voltage Vno generated by the ionisation chamber DET1 and applied tothe detector input of the detector integrated circuit IC4 drops belowthe reference voltage Vref level set at the sensitivity input. When thisoccurs, the alarm is triggered.

When the voltage applied to the first output rail 306 by the counterintegrated circuit IC3 ceases, the voltage on the first output rail 306drops to a relatively low potential so that the first detectortransistor TR2 switches off. With the timer capacitor C13 discharged,current flows from the supply rail 210 to the capacitor which begins tocharge. While the capacitor C13 is charging, the voltage applied to theionisation chamber DET1 remains low owing to the charging current beingdrawn by the capacitor. However, as the charge on the capacitorincreases, the voltage applied to the ionisation chamber rises. After aperiod of time, the voltage Vno generated by the ionisation chamber andapplied to the detector input of the detector integrated circuit IC4rises to a value above the reference level Vref set by the sensitivityinput. The alarm thus stops triggering.

The above described circuitry allows the testing of the alarm by meansof the energising and de-energising of the lighting circuit to which thealarm is connected, i.e. by the flicking of a light switch. It should benoted that, although the description makes reference to a process of“energising and de-energising”, this order of operation is not essentialand the circuit may be arranged to respond additionally or alternativelyto a “de-energising and re-energising” of the lighting circuit.

It will be understood that the testing operation effectively simulates asituation whereby smoke is detected by the ionisation chamber byreducing the voltage supplied to the ionisation chamber and hencereducing the voltage generated thereby below the sensitivity threshold.Thus, both the ionisation chamber and the detector integrated circuitIC4 is tested, rather than simply the alarm sounder as in manyconventional alarms.

It will be further understood that the capacitor C13 can act as a timerfor maintaining the alarm in a test state for a length of timedetermined by the time constant of the capacitor and associatedresistor. The alarm remains in a test state i.e. active until the chargeon the capacitor reaches a predetermined level, irrespective of whetheror not the first detector transistor TR2 is on, i.e. whether or not avoltage is still applied to the first output rail 306. The voltageapplied by the counter integrated circuit IC3 on the first output railmay thus be in the form of a pulse having a relatively short duration.The pulse must be applied for a duration which need only be long enoughto enable the capacitor C13 to discharge.

As described above, the voltage Vref applied to the sensitivity input ofthe detector integrated circuit IC4 determines the sensitivity thresholdat which the alarm triggers. The detector integrated circuit IC4 allowsthe sensitivity of the alarm to be adjusted to compensate for differentoperating conditions. Thus, for example, if the alarm were fitted near akitchen where low levels of smoke are common, the sensitivity of thealarm can be reduced (by reducing Vref) to ensure that only unusuallylarge volumes of smoke would trigger the alarm and thus reduce falsealarms.

The sensitivity threshold voltage is set by a plurality of resistorsR22, R23, R25 and R35 forming a potential divider to which thesensitivity input 404 is connected. The sensitivity input is alsoconnected, via a resistor R19 and a blocking diode D7, to the collectorof a second detector transistor TR1. The emitter of the second detectortransistor TR1 is connected to the earth rail 112 while the base isconnected, via a limiting resistor R15, to the second output rail 308.

If the lighting circuit is energised and de-energised twice within apredetermined time period determined by the time constant of the R-Ctimer circuit associated with IC2, the pulses on the input line 301 aredetected by IC2 which sends a reset control signal to the counterintegrated circuit IC3. On receiving the reset control signal, thecounter integrated circuit IC3 applies a voltage to the second outputrail 308 which is then applied to the base of the second detectortransistor TR1. The second detector transistor TR1 is thus switched on.

Current thus flows from the supply rail 210 through resistor R19 so thatthe voltage Vref applied to the sensitivity input 404 of the detectorintegrated circuit IC4 is pulled down to a relatively low potential.Decreasing the voltage Vref applied to the sensitivity input of thedetector integrated circuit IC4 has the effect of decreasing thesensitivity of the detector integrated circuit IC4. When the voltageVref applied to the sensitivity input drops below the voltage Vnoapplied to the detector input of the detector integrated circuit IC4,the falsely triggering alarm is effectively reset.

False triggering of smoke alarms is usually caused by the ionisationchamber detecting small amounts of smoke or other airborne particulateswhich results in the voltage Vno generated by the ionisation chamber andapplied to the detector input of the detector integrated circuit IC4being lower than the reference voltage Vref applied to the sensitivitythreshold. Reducing the voltage Vref decreases the sensitivity thresholdof the alarm. When the sensitivity threshold voltage Vref decreasesbelow the voltage Vno applied by the ionisation chamber DET1 to thedetector input, the alarm stops triggering. The alarm is thuseffectively reset.

The integrated circuit IC4 also has a low battery charge input and atthe same time at the same time as the voltage applied to the sensitivityinput is reduced, the voltage applied to the low battery charge inputalso reduces. This effectively increases the reference voltage for a“low battery” sensor in the detector integrated circuit IC whichsimulates a low battery condition. This is indicated by a once-perminute “chirp” from the alarm which thus has the dual role of indicatinga low battery charge (if occurring continuously) and warning of a lowsensitivity condition (if occurring for only a short time).

In addition to the above, the detection circuitry enables thesensitivity threshold value Vref to return from its lowered, resetposition to its normal position either by way of a step change or, morepreferably, by a gradual change or ramp back to the original level. Thisis achieved by means of a capacitor C8 connected between the limitingresistor R15 and the earth rail 112.

When the voltage is applied to the second output rail 308 by the counterintegrated circuit IC3 and transistor TR1 turns on, the capacitor C8charges. When the voltage applied to the second output rail 308 ceasesthe charge on capacitor C8 maintains transistor TR1 ON. However, thecapacitor C8 begins to discharge through a current limiting resistor R16and the voltage applied to the base of the second detector transistorTR1 decreases. As this voltage decreases, the second detector transistorTR1 switches from a conducting state to a substantially non-conductingstate. However, this change in state is gradual as the voltage appliedto the base decreases. Thus, the voltage applied to the sensitivityinput rises, thereby increasing the sensitivity of the detectorintegrated circuit IC4.

Thus, if the cause of the false alarm is smoke from cooking or otheractivities, this is unlikely to exceed the reduced sensitivity thresholdlevel and will gradually clear as the sensitivity of the alarmincreases. Advantageously, by the time that the normal sensitivitythreshold level is reached, the smoke is likely to have cleared.

It will be appreciated that the above described circuitry provides a fargreater level of safety for the user than achieved by existing systems.The ability to reset the alarm and reduce its threshold sensitivity bythe simple act of flicking a light switch eliminates the requirement ofexisting alarms for the battery to be removed or otherwise tamperedwith. In addition, in the event of a real fire after resetting of thealarm, the alarm is still operable and, even in the reduced sensitivitymode, is likely to trigger correctly, thereby advising the user of thereal emergency.

FIG. 6 illustrates an alternative form of charging circuit 600 for thealarm. The circuit is broadly similar to that of FIG. 2 and performs asimilar function. However, an important difference is that the circuitof FIG. 6 for goes the bridge rectifier BR1. Instead, the earth rail 112is formed by the neutral input PL2 so that the voltage on the positiverail 110 is only half-wave rectified. The value of the capacitor C2 isincreased to increase the smoothing applied to the half-wave rectifiedcurrent and an additional input capacitor C15 is connected, in parallelwith a plurality of series-connected resistors R1, R2, R3, to increasethe current limit through the circuit. The resistors R1, R2 and R3 serveto provide a discharge path for the capacitor C15 when the mains powersupply is switched off.

A light emitting diode LED1 is connected between the positive rail 110and the earth rail 112 to indicate when a voltage is being applied tothe inputs PL1, PL2, i.e. to indicate when the charging circuit isswitched on. Also, the voltage regulator IC1 of FIG. 2 is not includedin the charging circuit, being replaced by a resistor R47 and zenerdiode D4 combination.

FIG. 7 illustrates an alternative form of control circuit 700 for thealarm which has a logic circuit 702 and a signal conditioning circuit704. Again, the principle of operation of the circuit of FIG. 7 issimilar to that of FIG. 4. In this embodiment, however, additionalcircuitry is included to permit the use of a separate test/reset buttonSW2 on the alarm itself. This allows the alarm to be tested and/or reseteither by the light switch as described above, or by the push buttonSW2. When the circuit of FIG. 7 is used as the control circuit theterminal PL1 of the charging circuit 200 is connected to the live cablein the lighting circuit and not to the switch live side of the switch. Aseparate connection through the conditioning circuit 704 as describedbelow is made from the circuit of FIG. 7 to the switch live side of theswitch.

The push button SW2 is connected, via a parallel combination of acapacitor C17 and a resistor R55, to the DC supply of the supply rail210. When the push button SW2 is actuated to close the switch thevoltage applied to the trigger input of IC2 goes high. The triggervoltage then decays as the capacitor C17 is charged. Thus, a pulse isapplied to the input of IC2. When IC2 receives a preselected number ofpulses within a preselected time period it sends a control signal to IC3which then applies a voltage to output rail 306.

Actuating the push button SW2 a preset number of times over a presettime period causes the alarm to trigger in its test mode as describedabove with reference to FIGS. 4 and 5.

The control circuit of FIG. 7 also has a switched live input SL which isconnected to the light side of the light switch and goes live when thelight is switched on.

In the embodiment of FIGS. 2 to 5, when the light switch is ON thesignal actually applied to the trigger input of IC2 is a rectified butunsmoothed signal from the bridge rectifier BR1, i.e. a series ofpositive going pulses. Because the trigger input of IC2 responds tovoltage pulses applied thereto, the application of this signal to thetrigger input causes IC2 to generate an output pulse which iscontinuously refreshed so that the output of IC2 is permanently high.This is satisfactory in the embodiment of FIGS. 2 to 5 since the lightswitch can be switched on and off to simulate “pulses” applied to thetrigger input. Thus, for each ON/OFF flick of the light switch a singlepulse is generated by IC2. However, if this were the case in theembodiment of FIG. 7 then IC2 would be unable to distinguish the pulsegenerated by the push button SW2 from the train of pulses applied by theswitched live AC signal from the switched live input SL. This wouldresult in the push button SW2 being ineffective whilst the switched liveinput were energised i.e. whilst the light were switched on.

It is therefore advantageous to prevent continuous retriggering of IC2even whilst the switched live input SL is energised. In FIG. 7, theswitched live input is connected to the trigger input of IC2 via anumber of resistors R13 to R16, R56 and a reverse biased diode D7. Theanode of the diode D7 is additionally connected to the collector of atransistor TR13 whose emitter is connected to the earth rail 112. Thebase is connected, via a limiting resistor R54, to the junction betweena resistor R53 and a capacitor C16 which are connected in series betweenthe switched live input S and the earth rail.

With the switched live input SL deactivated (i.e. the light switch isoff), the voltage applied to the trigger input of IC2 is determined by apotential divider formed by a resistor R17 on the one hand and resistorsR56 and R48 on the other hand. R17 is chosen very much larger than bothR53 and R48 so that the voltage applied to the trigger input of IC2 islow. The transistor TR13 is switched off and so current from the batteryflows to the earth rail through R17, R48 and R56.

When the switched live input SL is energised, i.e. the light switch isswitched on, zener diode D6 clips the AC voltage to approximately 12V,effectively rectifying the AC voltage by clamping negative voltagesclose to earth potential. The voltage applied to the cathode of thediode D7 is greater than that applied to the anode of the diode D7 fromthe battery. The current from the battery is thus unable to flow throughthe diode D7 and so the voltage applied to the trigger input of IC2 israised approximately to the supply voltage, causing IC2 to generate asingle output pulse. This is used to set the alarm as described above.

However, when the switched live input S is energised, capacitor C16begins charging at a rate determined by the time constant of thecapacitor C16 and the resistor R53. When the charge on the capacitor C16reaches a predetermined level, transistor TR13 is switched on. Currentfrom the supply rail thus flows through the transistor TR13 to the earthrail which thereby pulls the voltage applied to the trigger input of IC2low. This voltage is then clamped low by the transistor TR13 until theswitched live input S is de-energised and the capacitor C16 hasdischarged. During this time, the push button SW2 can be used to test orreset the alarm as described above.

The duration of the output pulse generated by IC2 is such that thevoltage applied to the trigger input of IC2 is pulled low before thepulse ends.

While the lighting circuit to which the alarm is connected is energised,therefore, the alarm can be tested by means of the push button SW2.While the lighting circuit is off, the alarm can be tested both by thepush button SW2 and by the light switch. It will be understood that ifone wishes to test the alarm by means of the light switch when thelighting circuit is energised, the lighting circuit must first beswitched off, simply requiring an additional OFF operation of the lightswitch.

The alarm of the invention is able to be connected to one or moreadditional alarms so as to provide a network of alarms for use in abuilding or the like. The detector integrated circuit IC4 is providedwith a common input/output (I/O) pin for connection to a similar pin ona like detector integrated circuit via an input/output (I/O) line.Legislation in certain countries dictates that a relatively low voltageon the I/O line should be used to signal an emergency condition so thatif a short circuit occurs between the I/O line and, for example, theneutral cable or an earth cable, the alarm will default to the emergencycondition.

However, the detector integrated circuit IC4 is arranged to alarm when arelatively high voltage is applied to the I/O pin and, conversely,applies a relatively high voltage to the I/O pin if the ionisationchamber DET1 detects smoke locally. It is therefore necessary, in alarmsfor use in such countries, to provide an inverter circuit for invertingthe signal generated by the detector integrated circuit IC4 fortransmission on the I/O line and, equally, for inverting the signalreceived on the I/O line from a connected alarm. No inverting circuitrymay be required when the alarm is to be used in countries which do notcarry such legislation.

It will be understood that the system may be configured such that in theevent of a false alarm whereby all of the alarms are triggered, theinitially falsely triggered alarm can be reset using the techniquedescribed above. This will also reset the remaining alarms in thesystem. Importantly, however, the sensitivity threshold of the falselytriggered alarm will be reduced whilst those remaining alarms in thesystem will be unaffected and will retain their normal sensitivitythreshold levels. It will be appreciated that this adds a far greatersafety factor should a fire start elsewhere in a building and minimisesinconvenience to the user.

Referring to FIGS. 8 to 15, the alarm of the present invention isprovided advantageously with a unique design of housing or casing 500.Conventional ceiling-mounted alarms use a backing plate on which thedetection circuitry is mounted. The backing plate has an aperture forallowing the mains circuit power cable to be passed through and attachedto appropriate connectors provided in the detection circuitry.Additional apertures are provided as guides for screw holes to enablethe backing plate to be screwed to a ceiling fixture. Since the backingplate lies against the ceiling surface with the detector circuitrymounted directly beneath the backing plate within a cover, the alarm hasa certain depth which, if it could be reduced, would improve theaesthetics of the alarm.

The alarm of the present invention is conveniently provided with acircular housing which reduces the depth of the alarm. Specifically, thehousing 500 comprises a first backing plate 502 generally in the form ofan annular ring having a large internal aperture 504. The first backingplate 502 is arranged to be fixed to a ceiling or other fixture. Theinternal aperture 504 is conveniently used as a guide for the user tocut out the portion of the ceiling defined by the aperture and throughwhich the power cables will pass. The first backing plate also has atleast two clips 514 which are preferably equiangularly spaced about theperiphery of the plate and project radially inwardly from its innerface. They are raise relative to the rim of the plate in a directioninwardly of the housing.

A clip 520 (FIGS. 14 and 15) is provided on the first backing plate 502which is attached thereto by a weakened region so that the clip mayeasily be snapped off from the first backing plate, as described below.

A second backing plate 506 has a raised central portion 508 in which thesmoke detector circuitry 510 is seated and is mounted to the firstbacking plate 502 by means of clips 512 on the first backing plate orany other suitable means such that the raised central portion 508 liessubstantially flush with the first backing plate 502. The second backingplate also has clips 516 corresponding to clips 514 which are spacedabout the periphery of the plate and project radially inwardly from itsinner face towards the backing plate 502.

A cover portion 514 is mounted to either or both of the first and secondbacking plates 502, 506 for enclosing the circuitry 510 and improvingthe aesthetic appearance of the alarm. The alarm is considerably moreslim-line than existing alarms.

To install the alarm, the user fixes the first backing plate 502 to theceiling or other fixture using screws or the like. The user then cuts anaperture in the ceiling via the aperture 504 in order to access thecables from the lighting or ring circuit to which the alarm is to beconnected. The cables from the lighting or ring circuit are connectableto the alarm by means of a plug or connector 516 which engages with acorresponding socket on the alarm. To facilitate installation, the usermounts the plug 516 onto the clip 520 which holds the plug in positionwhile the users connects the cables from the mains circuit thereto. Theclip 520 has fingers 522 with end hooks 524 which clip over the plug 516to retain the plug. This enables the user to connect the cables to thelighting circuit without the risk of pulling the plug or cables backthrough the aperture in the ceiling. When the cables have been connectedproperly, the user detaches the plug from the clip 520 and then detachesthe clip 520 from the first backing plate 502. The plug 516 can then beengaged with the socket on the alarm.

Advantageously, the alarm is arranged so that, when the plug 516 andsocket are engaged, they lie substantially flush with the first backingplate 502, thereby reducing the depth of the alarm. It will beunderstood that the terms “plug” and “socket” are used arbitrarily andthat the plug may be located on the alarm and the socket used forconnection to the cables of the mains circuit.

To connect the second backing plate 506 to the first backing plate 502the former is offered up to the first backing plate with the clips 516adjacent clips 514. The second backing plate 506 is then rotated toslide the clips 516 behind the clips 514 and secure the two platestogether. A stop can be provided on one or both backing plates toprevent further rotation of the second backing plate 506 relative to teefirst when the clips are fully engaged. The dimensions of the clips andtheir arrangement is such that a secure and firm connection is madebetween the two backing plates.

FIGS. 11 to 13 show a preferred form of connection arrangement 550. Thearrangement has a push-to-break switch 552 which is actuated by anactuator 554 in the form of a spigot or lever accessed from outside thealarm housing. The lever is generally L-shaped and pressed from the bodyof the second backing plate 506 with one arm of the “L” extending in theplane of the plate and the other arm 562 extending away from the firstbacking plate into the body of the housing and contacting a switch arm556. The switch arm 556 has a depending flange 558 at one end which ismounted on a circuit board and connected to the gate of TR3 whilst theother, free end of the switch arm rests on a pad or contact 560 which iselectrically connected to the source of TR3. The switch arm is either aresilient arm which is self biased against the pad or is provided withbiasing means such as a coil spring.

The second arm of the lever contacts the free end of the switch arm suchthat in the normal rest attitude of the lever 554 the free end of theswitch arm contacts the pad and shorts the source and gate of TR3together to disable the alarm. The lever 554 also has a spigot or raisedportion formed at the junction of the two arms of the “L”, the spigotbeing raised above the surrounding surface of the plate 506. When thesecond backing plate 506 is offered to the first backing plate androtated into engagement, a cooperating portion (such as a raised portionor ramp-like portion) engages the spigot 556 to depress the latter anddisengage the free end of the switch arm from the pad 560 and arm thealarm.

In one embodiment, a small, clearly labelled hole 564 is provided on thecasing of the alarm. The hole has a metallised internal surface and iselectrically connected to the pad 560. Thus, if the switch arm, forwhatever reason, fails to contact the pad 560 when the second backingplate 506 is disengaged from the plate 502 a small metal wire, forexample a bent paperclip, can be inserted through the hole to short theswitch arm to the pad and disconnect the battery and silence the alarm.

Alternatively, a push button switch, accessible directly or through ahole by means of a narrow object such as a pencil, a pin or a tooth picketc., could be employed to enable the user manually to disconnect thepower supply from the detection circuitry.

In one embodiment, the switch is arranged so that the power supply, isdisconnected from the detection circuitry by default and actuation ofthe switch causes the power supply to be connected to the detectioncircuitry. The switch may be actuated by means of a pin located on acover or housing portion arranged to fit over the alarm once installed.Fitting of the cover to the alarm causes the pin to engage with theswitch thereby re-connecting the power supply to the detectioncircuitry.

Referring now to FIGS. 16 to 18 these show three ways in which the alarmcan be connected to a lighting circuit.

In FIG. 16, the live and neutral terminals PL1, PL2 are connected to aconsumer board 800 or other power distribution board. This is a standardconfiguration where the switch live SL terminal is not used. It will beappreciated that for this arrangement an alarm with the control circuitof FIG. 7 is used and the setting and resetting is achieved by use ofthe switch SW2 on the alarm housing. The alarm is wired to permanentlive and neutral cables of a ring main circuit or similar. The mainscircuit powers the alarm at all times except in the event of, forexample, a power cut whereby the alarm is powered by the battery actingas a back-up power supply.

In FIG. 17, the live and neutral terminals PL1, PL2 are connected to theconsumer board 800 or other power distribution board or to a ceilingrose for a light The switch live terminal SL is connected to the lightside of the light switch. In this arrangement an alarm with the controlcircuit of FIG. 7 is used and the setting and resetting is achievedeither by use of the light switch or by use of the switch SW2 on thealarm housing. Here, the alarm is wired to permanent live and neutralcables and also to a switched live cable. The alarm is powered at alltimes by the mains circuit but can be tested and/or reset by the pushbutton switch SW2 and/or the light switch.

In FIG. 18, the live terminal PL1 and switch live terminal SL areconnected together and to the switched live cable of a lighting circuit.Here, the light switch can be used to test/reset the alarm in additionto the push button switch SW2, where present, and when the lightingcircuit is de-energised (i.e. the light is not in use), the alarm ispowered by the battery.

The circuits shown in the accompanying drawings may be modified toachieve variations on the functions described. For example, the numberof operations of the push button switch SW2 for a given function can bematched to the number of operations of the light switch. Variousadditional features can be added and activated by increasing the numberof operations of the push button switch SW2 and/or the light switch. Thelight switch operation can be set to detect “off-on-off” sequences inaddition to or alternatively of “on-off-on” sequences. Advantageously,only a single push button switch SW2, which could be any suitable formof switch, and/or a single light switch is needed to operate all of thefunctions of the alarm.

An interconnect for communication between two or more alarms can beincluded but is entirely optional.

In a further embodiment, the alarm includes a relay or other suchswitching device which, when the alarm is triggered, connects thepermanent live cable of the power supply (where present) to a switchedlive cable of a lighting circuit. This provides the advantageous effectthat, when the alarm is triggered, the light connected to the switchedlive cable is automically illuminated. FIG. 19 shows a modification tothe alarm circuit to achieve this. In FIG. 19 the charging circuit 100is connected to the live and neutral of a lighting circuit power supply.The signal conditioning circuit 704 has as an input the switched liveoutput of the light switch S and is connected to the logic circuit 792as described earlier with reference to FIG. 7. In addition, the live ofthe power supply is connected to the switched live SL input of thecircuit 704 by way of a power conditioning circuit 710 and a relay 712which is conveniently a solenoid operated 240 v relay. The relay 712 isactuated by a signal from the detection circuit 400 when the alarm isactuated in order to switch on the light LB when the latter is off. Thepower conditioning circuit 710 is at its simplest a diode 714.

When an alarm condition is present, the relay 712 is actuated to connectthe live rail to the light LB by way of the diode 714.

When a test or reset signal is applied to the signal conditioningcircuit by flicking the switch S on and off one or more times the ACmains signal applied to the circuit 704 via the relay 702 is preventedfrom triggering the alarm. The diode 714 provides half waverectification of the AC mains to allow only negative going pulsesthrough the relay 714 to the signal conditioning circuit 704 when therelay is closed. However, the circuit 704 only senses positive goingpulses, as a result of which the mains pulses which power the lightthrough the relay 712 do not trigger the alarm.

In addition, all interconnected alarms and lights could be switched onso that, in the event of a fire in a tall building such as athree-storey town house, an escape route would be illuminated thereby.

It will be appreciated that the present invention provides a significantimprovement over existing alarms. It will be understood that the variousfeatures of the alarm described above are not mutually inclusive and canbe used independently of the other features if required. For example,the casing/housing described for the alarm may be applicable to alarmsother than those connectable to a lighting circuit.

The disconnect circuit may find application in devices other than smokealarms or may be modified for use with smoke alarms such thatinstallation of the alarm or connection to the mains circuitautomatically reconnects the power supply to the detection circuitry.This may be particularly the case for alarms such as those described inco-pending application No. WO 00/58924, the contents of which are hereinincorporated by reference.

1-39. (canceled)
 40. An alarm for detecting radiation and/or pollutantssuch as smoke, carbon monoxide or the like having: a housing means; analarm circuit including detection means (DET1) for detecting saidradiation and/or pollutants; first electrical connection means (PL1,PL2) connectable to an external power supply for supplying power to saidalarm circuit; and control means responsive to receipt of a preselectednumber of pulses over a preselected time period to apply a presetcontrol signal to said alarm circuit; wherein said alarm circuit isresponsive to said preset control signal to reset or test said alarm independence on said preset control signal.
 41. An alarm as claimed inclaim 40 wherein: said control means is responsive to the energising andde-energising of the external power supply said preselected number oftimes over said preselected time period to apply said preset controlsignal to said alarm circuit.
 42. An alarm as claimed in claim 40wherein: said alarm has first switch means (SW2) actuable by a user togenerate a respective pulse for each actuation thereby to apply a userselected number of pulses to said control means; and said control meansis responsive to receipt of said preselected number of said pulses oversaid preselected time period to apply a preset control signal to saidalarm circuit.
 43. An alarm as claimed in claim 42 wherein said firstswitch means is mounted on said alarm housing.
 44. An alarm as claimedin claim 42 wherein said first switch means (SW2) is mounted remote fromsaid alarm housing.
 45. An alarm as claimed in claim 43 wherein saidfirst switch means (SW2) is adapted for connection to a switch live sideof a switch for a lighting circuit.
 46. An alarm as claimed in claim 40wherein: said alarm has second electrical connection means (SL) forconnection to a switch live side of a switch for a lighting circuit; andwherein said second electrical connection means is operable to receivepulses caused by user actuation of said switch between its on and offstates and apply said pulses to said control means thereby to cause apreset control signal to be applied to said alarm circuit in response togeneration of said preselected number of pulses over said preselectedtime period.
 47. An alarm as claimed in claim 40 further comprisingswitch means (RL1) for an external light source (LB) and actuable inresponse to generation of a preselected control signal to energise saidlight source.
 48. An alarm as claimed in claim 40 further comprising arelay (RL1) and a light source (LB) wherein said relay is actuable inresponse to generation of a preselected control signal to energise saidlight source.
 49. An alarm as claimed in claim 40 wherein when saidpreselected number of pulses over said preselected time period is one,said control means is operable to apply a preset control signal to saidalarm circuit thereby to reset said alarm.
 50. An alarm as claimed inclaim 40 wherein when said preselected number of pulses over saidpreselected time period is one, said control means is operable to applya preset control signal to said alarm circuit thereby to test saidalarm.
 51. An alarm as claimed in claim 40 wherein when said preselectednumber of pulses over said preselected time period is two, said controlmeans is operable to apply a preset control signal to said alarm circuitthereby to test said alarm.
 52. An alarm as claimed in claim 50 whereinwhen said preselected number of pulses over said preselected time periodis two, said control means is operable to apply a preset control signalto said alarm circuit thereby to reset said alarm.
 53. An alarm circuitas claimed in claim 40 wherein said alarm circuit comprises means (TR1)for reducing the sensitivity of said detection means (DET1).
 54. Analarm as claimed in claim 53 wherein said means (TR1) for reducing thesensitivity of said detection means (DET1) is operable in response togeneration of a reset control signal by said control means to reduce thesensitivity of said detection means (DET1) for a preselected time periodthereby to reset said alarm.
 55. An alarm circuit as claimed in claim 40wherein said alarm circuit comprises means (TR2) for increasing thesensitivity of said detection means (DET1).
 56. An alarm as claimed inclaim 54 wherein said means (TR2) for increasing the sensitivity of saiddetection means (DET1) is operable in response to generation of a testcontrol signal by said control means to increase the sensitivity of saiddetection means (DET1) for a preselected time period thereby to testsaid alarm.
 57. An alarm as claimed in claim 40 further comprising: abattery (B1) for supplying power to said alarm.
 58. An alarm as claimedin claim 57 further comprising: a charging circuit including said firstelectrical connection means (PL1, PL2) for supplying power to a powerrail for said alarm and for charging said battery.
 59. An alarm asclaimed in claim 57 further comprising an isolating means for selectablyelectrically disconnecting said battery from said alarm thereby tominimise leakage from said battery when said alarm is inactive.
 60. Analarm as claimed in claim 58 wherein: said isolating means comprises asecond switch means (TR3) in said power rail switchable between a first,conducting state connecting said battery (B1) to said alarm and asecond, non-conduction state disconnecting said battery from said alarm.61. An alarm as claimed in claim 60 wherein said charging circuitcomprises a third switch means (TR5) switchable between a first,conducting state and a second, non-conducting state in dependence on thevoltage on said power rail; and wherein: when said third switch means(TR5) is in said first, conducting state said third switch means (TR5)is operable to retain said isolating second switch means (TR3) in itsconducting state; and when said third switch means (TR5) is in saidsecond, non-conducting state the state of said third switch means (TR5)is dependent on the voltage on said power rail such that said secondswitch means (TR3) is non-conducting in response to said voltage on saidpower rail being below a preselected value indicating a low batterycharge, thereby to disarm said alarm during charging of said battery(B1).
 62. An alarm as claimed in claim 61 further comprising adisconnect means actuable to switch said switch means (TR3) into itsnon-conducting state thereby disabling said switch means and preventingactuation of said alarm.
 63. An alarm as claimed in claim 62 whereinsaid disconnect means comprises button means movable between a first,OFF position wherein said switch means (TR3) is rendered non-conductingand a second, ON position wherein said switch means (TR3) is enabled.64. An alarm as claimed in claim 62 wherein: said switch means (TR3) isa multi electrode semiconductor device having a control electrode forcontrolling conduction between further electrodes thereof; and saidbutton means is movable into its first, OFF position to vary thepotential on said control gate means thereby to render said switch means(TR3) non-conducting.
 65. An alarm as claimed in claim 63 wherein: saidhousing comprises: a first backing plate for mounting on a surface; asecond backing plate detachably mountable on said first backing plate;and a cover means for covering said backing plates; and wherein thearrangement of said disconnect means is such that engagement of saidsecond backing plate on said first backing plate moves said disconnectmeans into its second, ON position thereby to enable said switch means(TR3) and disengagement of said second backing plate from said firstbacking plate moves said disconnect means into its first, OFF positionthereby to disable said switch means (TR3).
 66. An alarm as claimed inclaim 59 further comprising indicator means (LED1) operable in responseto power on said voltage rail downstream of said isolating means toindicate that said alarm is enabled.
 67. An alarm for detectingradiation and/or pollutants such as smoke, carbon monoxide or the likehaving: a housing means; an alarm circuit including detection means(DET1) for detecting said radiation and/or pollutants; first electricalconnection means (PL1, PL2) connectable to an external power supply forsupplying power to said alarm circuit; and switch means (RL1) for alight source (LB), said switch means being actuable in response totriggering of said alarm to energise said light source.
 68. An alarm asclaimed in claim 67 wherein said switch means comprises a relay (RL1)and said light source (LB) is external to said alarm.
 69. An alarm asclaimed in claim 67 wherein said light source is mounted in said alarm.70. An alarm for detecting radiation and/or pollutants such as smoke,carbon monoxide or the like having: a housing means; an alarm circuitincluding detection means (DET1) for detecting said radiation and/orpollutants; a battery (B1) for supplying power to said; and an isolatingmeans for selectably electrically disconnecting said battery from saidalarm thereby to minimise leakage from said battery when said alarm isinactive.
 71. An alarm as claimed in claim 70 further comprising: firstelectrical connection means (PL1, PL2) connectable to an external powersupply for supplying power to said alarm circuit; and a charging circuitincluding said first electrical connection means (PL1, PL2) forsupplying power to a power rail for said alarm and for charging saidbattery.
 72. An alarm as claimed in claim 70 wherein: said isolatingmeans comprises a second switch means in said power rail switchablebetween a first, conducting state connecting said battery (B 1) to saidalarm and a second, non-conduction state disconnecting said battery fromsaid alarm.
 73. An alarm as claimed in claim 71 wherein said chargingcircuit comprises a third switch means (TR5) switchable between a first,conducting state and a second, non-conducting state in dependence on thevoltage on said power rail; and wherein: when said third switch means(TR5) is in said first, conducting state said third switch means (TR5)is operable to retain said isolating second switch means (TR3) in itsconducting state; and when said third switch means (TR5) is in saidsecond, non-conducting state the state of said third switch means (TR5)is dependent on the voltage on said power rail such that said secondswitch means (TR3) is non-conducting in response to said voltage on saidpower rail being below a preselected value indicating a low batterycharge, thereby to disarm said alarm during charging of said battery(B1).
 74. An alarm as claimed in claim 72 further comprising adisconnect means actuable to switch said switch means (TR3) into itsnon-conducting state thereby disabling said switch means and preventingactuation of said alarm.
 75. An alarm as claimed in claim 74 whereinsaid disconnect means comprises button means movable between a first,OFF position wherein said switch means (TR3) is rendered non-conductingand a second, ON position wherein said switch means (TR3) is enabled.76. An alarm as claimed in claim 74 wherein: said switch means (TR3) isa multi electrode semiconductor device having a control electrode forcontrolling conduction between further electrodes thereof; and saidbutton means is movable into its first, OFF position to vary thepotential on said control gate means thereby to render said switch means(TR3) non-conducting.
 77. An alarm as claimed in claim 75 wherein: saidhousing comprises: a first backing plate for mounting on a surface; asecond backing plate detachably mountable on said first backing plate;and a cover means for covering said backing plates; and wherein thearrangement of said disconnect means is such that engagement of saidsecond backing plate on said first backing plate moves said disconnectmeans into its second, ON position thereby to enable said switch means(TR3) and disengagement of said second backing plate from said firstbacking plate moves said disconnect means into its first, OFF positionthereby to disable said switch means (TR3).
 78. An alarm as claimed inclaim 71 further comprising indicator means (LED1) operable in responseto power on said voltage rail downstream of said isolating means toindicate that said alarm is enabled.